With reference to FIG. 1, a ducted fan gas turbine engine is generally indicated at 10 and has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, an intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
During operation, air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
As shown in FIG. 2, the propulsive fan 12 typically comprises a fan disc 24 carrying a plurality of circumferentially-spaced, radially outwardly-extending fan blades 25. The fan disc 24 has a plurality of circumferentially arranged slots 26 provided in its rim of the fan disc. Each fan blade 25 has a root 27 and the root 27 of each fan blade 25 is arranged is a corresponding one of the slots 26 in the rim of the fan disc 24. The roots 27 of the fan blades 25 are generally dovetail-shaped in cross-section, and the slots 26 in the fan disc 24 are correspondingly shaped to receive the roots 27 of the fan blades 25.
The fan blades 25 in a gas turbine engine are relatively large, particularly in aerospace applications. When the engine is running, the blades 25 are centrifuged outward so that the dovetail roots 27 of the fan blades 25 are held in contact with, and retained by, correspondingly-shaped faces of the slots 26. However, when the engine spools down, the centrifugal force is overcome by the weight of the fan blade 25 below a certain speed. Hence, at low rotation speeds, the fan blade roots 27 tend to fall loose and move relative to the slots 26. This unconstrained movement can lead to fretting between the fan blade roots 27 and slots 26, which causes loss of the lubricant between the mating faces. There is therefore a risk of damage, if not actual damage, every time the engine is shut down or started. Similar movement, with similar consequences, may be caused by the windmilling—caused by the wind blowing through the engine—that is commonly seen when an aircraft is parked on the ground.
Fan blades 25 are typically chocked by inserting a spring-carrying slider 28 as shown in FIG. 3. The spring 29 is of a leaf design and fits to the slider 28 before being driven (using an impact tool) between the blade 25 and the base of the slot 26, such that the spring is compressed against the root 27.
Damage and/or marking to either or both of the slider 28 and the blade root 27 may occur as the slider is fitted under impact and the spring 28 surface is forced against the blade root 27. This damage is especially prevalent when the blade 25 is formed of composite material which has a lower resistance to damage and crushing stress.
Another problem with the known devices is that the spring 29 is fixed and thus radial force applied to the blade root 27 depends only on the distance between the blade root 27 and the radially inner surface of the slot 26. This distance is subject to manufacturing tolerances and thus the radial force applied by the slider 28 is subject to unpredictability. This means that the slider 28 is designed to cater for the anticipated maximum distance between the blade root 27 and the radially inner surface of the slot. As a result, unnecessary overloading on the blade root 27 may occur in the many instances where the distance between the blade root 27 and the radially inner surface of the slot 26 is less than the anticipated maximum. There remains a need for an improved retention device which at least ameliorates the above described problems.